allow to specify disabled message payload in mirror.py

[vladak]

Is your feature request related to a problem? Please describe.
The mirror.py script can execute a script or do a RESTful call for disabled projects. In the latter case more flexibility in the payload would be handy so that the users can determine the cause easily from the UI.

Describe the solution you'd like
If a project is disabled the value of the disabled property can be either boolean or string so the RESTful call will either use default message or custom, respectively.

Example:

projects:
  foo:
    # no longer active but used to support customers
    disabled: true
  bar:
    disabled: disabled, upstream not reachable (as of 02/2020)

Alternatives
Use the default payload as a prefix.

[vladak]

fixed via #3135